Aerodynamic performance of autorotating seeds: scaling by size

This study investigates the aerodynamics of a bio-inspired samara seed through high-fidelity numerical simulations, employing an overset mesh method to fully resolve its six-degree-of-freedom (6-DOF) motion. Coupled fluid and rigid body dynamics was solved using OpenFOAM v2406. A rigid 3D-printed seed prototype reproducing the samara of Acer campestre and its geometrically scaled versions (0.5x and 2x) were analyzed to explore the effects of scaling on passive flight dynamics. The simulations captured the full 6-DOF behavior, including the transition from uniformly accelerated vertical free-fall to steady autorotation. Key aerodynamic quantities such as descent velocity, angular velocity, coning and pitch angles, and the surrounding flow field structure were evaluated and compared. Simulation results are found to agree with scaling laws derived from the literature. Autorotation was found to be robust across scales, but strongly dependent on drop height and aerodynamic efficiency. The larger prototype (2x) exhibited the highest aerodynamic performance, while the small seed (0.5x) showed a reduced lift and, consequently, a comparatively higher descent velocity. Moreover, the 2x prototype, provided a greater surface area, thus offering potential functional benefits for applications to environmental sensing. Flow visualizations confirmed the formation of coherent leading-edge vortices, which contribute to lift generation and flight stability. The drop height necessary to establish steady autorotation increases with the size of the seed. These results suggest the existence of practical and biological limits for effective autorotational flight and offer design insights for passive bio-inspired flying systems that balance scalability, deployment constraints, and aerodynamic performance.